Single layer fuel lines and vapor return lines manufactured from synthetic materials such as polyamides have been proposed and employed in the past. Fuel lines employing such materials generally have lengths of at least several meters. It is important that the line, once installed, not materially change during the length of operation, either by shrinkage or elongation or as a result of the stresses to which the line may be subject during use.
It is also becoming increasingly important that the lines employed be essentially impervious to hydrocarbon emissions due to permeation through the tubing. It is anticipated that future Federal and state regulations will fix the limit for permissible hydrocarbon emissions due to permeation through such lines. Regulations which will be enacted in states such as California will fix the total passive hydrocarbon emission for a vehicle at 2 g/m.sup.2 per 24 hour period as calculated by evaporative emission testing methods such as those outlined in Title 13 of the California Code of Regulations, section 1976, proposed amendment of Sep. 26, 1991. To achieve the desired total vehicle emission levels, a hydrocarbon permeation level for the lines equal to or below 0.5 g/m.sup.2 per 24 hour period would be required. Finally, it is also imperative that the fuel line employed be impervious to interaction with corrosive materials present in the fuel such as oxidative agents and surfactants as well as additives such as ethanol and methanol.
Various types of tubing have been proposed to address these concerns. In general, the most successful of these have been co-extruded multi-layer tubing which employ a relatively thick outer layer composed of a material resistant to the exterior environment. The innermost layer is thinner and is composed of a material which is chosen for its ability to block diffusion of materials such as aliphatic hydrocarbons, alcohols and other materials present in fuel blends, to the outer layer. The materials of choice for the inner layer are polyamides such as Nylon 6, Nylon 6.6, Nylon 11 and Nylon 12.
Alcohol and aromatic compounds in the fluid conveyed through the tube diffuse at different rates through the tubing wall from the aliphatic components. The resulting change in the composition of the liquid in the tubing can change the solubility thresholds of the material so as, for example, to be able to crystalize monomers and oligomers of materials such as Nylon 11 and Nylon 12 into the liquid. The presence of copper ions, which can be picked up from the fuel pump, accelerates this crystallization. The crystallized precipitate can block filters and fuel injectors and collect to limit travel of the fuel-pump or carburetor float as well as build up on critical control surfaces of the fuel pump.
In U.S. Pat. No. 5,076,329 to Brunnhofer, a five-layer fuel line is proposed which is composed of a thick corrosion-resistant outer layer formed of a material known to be durable and resistant to environmental degradation such as Nylon 11 or Nylon 12. The tubing disclosed in this reference also includes a thick intermediate layer composed of conventional Nylon 6. The outer and intermediate layers are bonded together by a thin intermediate bonding layer composed of a polyethylene or a polypropylene having active side chains of maleic acid anhydride. A thin inner layer of after-condensed Nylon 6 with a low monomer content is employed as the innermost region of the tubing. The use of Nylon 6 as the material in the inner fluid contacting surface is designed to eliminate at least a portion of the monomer and oligomer dissolution which would occur with Nylon 11 or Nylon 12. The thin innermost layer is bonded to the thick intermediate layer by a solvent blocking layer formed of a copolymer of ethylene and vinyl alcohol with an ethylene content between about 30% and about 45% by weight. The use of a five layer system was mandated in order to obtain a tubing with the impact resistance of Nylon 12 with the low monomer/oligomer production of Nylon 6. It was felt that these characteristics could not be obtained in a tubing of less than five layers.
In U.S. Pat. No. 5,038,833 also to Brunnhofer, a three-layer fuel line without the resistance to monomer/oligomer dissolution is proposed in which a tube is formed having a co-extruded outer wall of Nylon 11 or Nylon 12, an intermediate alcohol barrier wall formed from an ethylene-vinyl alcohol copolymer, and an inner water-blocking wall formed from a polyamide such as Nylon 11 or Nylon 12. In DE 40 06 870, a fuel line is proposed in which an intermediate solvent barrier layer is formed of unmodified Nylon 6.6 either separately or in combination with blends of polyamide elastomers. The internal layer is also composed of polyamides, preferably modified or unmodified Nylon 6. The outer layer is composed of either Nylon 6 or Nylon 12.
Another tubing designed to be resistant to alcoholic media is disclosed in UK Application Number 2 204 376 A in which a tube is produced which has an thick outer layer composed of 11 or 12 block polyamides such as Nylon 11 or Nylon 12 which may be used alone or combined with 6 carbon block polyamides such as Nylon 6 or 6.6 Nylon. The outer layer may be co-extruded with an inner layer made from alcohol-resistant polyolefin co-polymer such as a co-polymer of propylene and maleic acid.
Heretofore it has been extremely difficult to obtain satisfactory lamination characteristics between dissimilar polymer layers. Thus all of the multi-layer tubing proposed previously has employed polyamide-based materials in most or all of the multiple layers. While many more effective solvent-resistant chemicals exist, their use in this area is limited due to limited elongation properties, strength and compatibility with Nylon 11 and Nylon 12.
In order to overcome these problems, multi-layer tubing material employing chemically different layers has been proposed in co-pending application Ser. Nos. 07/897,302, 07/897,376 and 07/896,824 to Noone and Mitchell, the inventors of the present invention. These tubing materials generally employ an outer polyamide layer bonded to an inner hydrocarbon resistant layer by means of a suitable intermediate bonding layer. While such materials do provide the desired characteristics of resistance to hydrocarbon permeation, the tubing produced is generally straight material which is difficult to successfully bend to conform to the contours in an automotive vehicle.
In most automotive applications, the tubing employed must be capable of bending to a variety of angles throughout its length to conform to the layout and the space requirements in the specific vehicle design. Various polymeric materials possess significant elastic memories which makes it difficult to successfully bend pieces of tubing into the permanent shape or contours necessary in the particular automotive application. Other polymeric materials are too rigid so that bends introduced into the material will cause crimping; thereby restricting flow therethrough and can experience significant reductions in its useful life due to fatigue and stress at or near the bend region. Furthermore bending previously known tubing can cause the differing layers to delaminate or fail due, in part, to the fact that the various layers each have very different elasticity and fatigue characteristics.
In order to obviate this problem, it has been proposed that conventional mono-layer tubing be corrugated at the appropriate bend regions. The bend region may include a plurality of annularly oriented accordion-like pleats which permit the region in which the pleats are located to be bent without constricting the interior opening or posing undue stress on the tubing material. This is accomplished by compressing one side of the each of the annular pleats in on themselves while the opposing side of can be extended outwardly from one another to accommodate the necessary angular contour. Heretofore no corrugated multi-layer tubing has been produced which incorporates chemically different layer materials in a single uniformly laminated wall.
Additionally no corrugated tubing has been produced or suggested which incorporates multiple layers of polymeric material having differing chemical properties. Without being bound to any theory, it is believed that conventional extrusion and tube forming processes are incapable of producing such material; particularly corrugated material having wall thicknesses below about 0.75 mm.
It would be desirable to provide a tubing material which could be employed in motor vehicles which would be durable and prevent or reduce permeation of organic materials therethrough. It would also be desirable to provide a tubing material which would be essentially nonreactive with components of the liquid being conveyed therein. It would also be desirable to provide a tubing material which exhibits these characteristics which has localized or overall areas of corrugation.